The present invention relates to a Faraday rotation single-mode fiber having a high Verdet constant for an optical isolator.
The Faraday effect is a phenomenon that the plane of polarization of a linear polarized light rotates when the light is propagated through a transparent medium placed in a magnetic field and in parallel with the magnetic field, and the effectiveness varies with the strength of the magnetic field, the Verdet constant inherent to the medium and the light path length. A material having a high Verdet constant is utilized for measurement of magnetic field (electric current) or as a Faraday rotator of an optical isolator. An optical isolator comprises a Faraday rotator to rotate by 45.degree. the plane of polarization, a magnet for application of magnetic field or a solenoid, and a polarizer and an analyzer. Conventional optical isolators have all been of the bulk type that no fiber is used. In the fields of optical fiber communication and optical fiber utilizing measurement in which a marked advance has been made in recent years, the necessity of using an optical isolator of the fiber type in the coupling part of a semiconductor laser and a fiber or in the intermediate part of a fiber and a fiber, for example, has been increased.
A fiber type optical isolator in which a fiber is used as a Faraday rotator exhibits the function as an isolator while confining light in the fiber and thus does not need couplings using lenses. Accordingly, the fiber type optical isolator has advantages over the conventional bulk type optical isolator in that the size is small, the weight is light and it is not easy to disorder optical arrayment by external disturbance. This fiber type optical isolator has been studied in Tohoku University, Japan, as described in K. Shiraishi et al., Applied Optics, Vol. 23, No. 7, pp. 1103-1106 (Apr. 1, 1984). In the fiber type optical isolator developed in Tohoku University, as a Faraday rotator, a single-mode fiber having a Verdet constant at
a wavelength of 633 nm of -0.25 min/Oe.multidot.cm is used.
The fiber used in the fiber type optical isolator of Tohoku University is a single-mode fiber which has the highest Verdet constant at the present time ("FR-5", produced by HOYA CORPORATION). For this fiber, however, an HL product (strength of magnetic field .times. light path length) of 10.8 KOe.multidot.cm is needed to rotate the plane of polarization by 45.degree., and thus the magnet used is limited in its miniaturization. Accordingly, to more reduce the size of the optical isolator, a single-mode fiber having a higher Verdet constant is needed.